The invention relates to a swivel comprising an outer annular wall and an inner annular wall coaxial with the outer wall, the outer and inner walls defining a toroxc3xafdal chamber and an upper and a lower gap between their peripheral opposing faces. The upper and/or lower gap has a first section extending axially from the toroxc3xafdal chamber, a second section extending generally transversely to the first section, and a third section extending at least partly substantially parallel to the first section. A seal is placed in the second or the third section of the upper and/or lower gap, the seal having two flexible legs arranged in a generally u-shaped or v-shaped orientation. During use, the gap near the area of the seal is filled with a sealing fluid, such as oil. A fluid duct extends through the outer or inner annular wall to the gap and has an outlet in the gap section between the seal and the toroxc3xafdal chamber, and means for transporting a flushing substance, such as oil, under pressure through the duct to the toroxc3xafdal chamber.
From WO 98/31963 a toroxc3xafdal swivel is known for transfer of hydrocarbons from a subsea oil well to a weathervaning floating production storage and offloading vessel (FPSO). In the toroxc3xafdal chamber that is enclosed by the stationary inner wall and rotating outer wall, the hydrocarbons are distributed towards the outlet in the rotating outer wall part. Within the toroxc3xafdal chamber pressures of 150-300 bar prevail tending to force the outer and inner walls apart. Due to pressure fluctuations, the gap between the walls will vary in width and may become widened such that the seals in the gap can be extruded from the gap. In order to construct a gap which is reduced in width upon internal pressurisation of the toroxc3xafdal chamber, a specific configuration of the gap between the swivel walls is disclosed in WO 98/31963 in the name of the applicant. According to the teachings of the above patent application, the outer wall comprises L-shaped or bevelled parts extending radially inwards with respect to the inner swivel wall, which comprises complementary L-shaped or sloping parts extending radially outwards. When the outer swivel wall is displaced in a radially outward direction due to the pressure in the toroxc3xafdal chamber, the L-shaped elements of the inner and outer walls, between which the extrusion gap is defined, are pressed together such that the width of the gap decreases.
With the known construction, a narrow gap between both walls can be obtained using relatively large tolerances upon manufacturing. However, the known system may give rise to problems especially when used in transporting gas or mixtures of oil and gas, or oil and water. For their proper functioning, the seals in the gap must at all times be surrounded by oil. If an oil-gas mixture or an oil-water mixture (there is always some gas and water in crude oil) enters the toroxc3xafdal chamber, gas will be released from the oil and travels upwards towards the upper gap and may expel oil from the seal area, whereas water will travel towards the lower seal area and expel the oil in which the lower seal is immersed. This gives rise to seal wear and improper seal functioning which will lead to leakage of hydrocarbons via the extrusion gap.
In another known swivel construction, having the features according to the preamble of claim 1, an isolation seal, or silt seal is located between the dynamic, or primary seals and the toroxc3xafdal chamber. On one side of the isolation seal an oil pressure duct ends in the extrusion gap to pressurise the isolation seal and to create substantially equal pressures on each side of the isolation seal. The purpose of the isolation seal is to contain the oil around the primary seals, an to prevent the oil from running freely into the toroxc3xafdal chamber upon tilting of the swivel. The isolation seal can be of a relatively light construction compared to the primary seal and has its flexible legs oriented towards the toroxc3xafdal chamber, or can be formed by a solid ring of PTFE-material. Furthermore, the isolation seal prevents the ingress of contaminating substances such as sand, to the primary seals. In order to remove contaminations from the seal area, it is known to flush these seal by injecting oil into the extrusion gap via the oil duct, which oil is passed along the isolation seal at an overpressure towards the toroxc3xafdal chamber. Also in this construction, the ingress of gas or water along the isolation seal, towards the primary seal can lead to malfunctioning of the primary seal. In case the oil is removed from the area of the primary seal it can, due to the large pressure difference across said seal, be subject to increased wear, bursts or cracks and consequent leakage.
It is an object of the present invention to provide a swivel in which the primary seal can be properly maintained in a clean oil environment and in which gas escaping from the oil or water separated from oil is prevented from expelling oil from the seal areas and does not impair the functioning of the seal.
Thereto the swivel according to the present invention is characterised in that the primary seal is placed in the second gap section, or in the third gap section at a position (H) below the outer part of the first gap section, the outlet of the fluid duct being placed in the second gap section or in the third gap section.
It was found that by placing the primary seal that is located closest to the toroxc3xafdal chamber, not further away from the toroxc3xafdal chamber than the outer end of the first, vertically extending section of the extrusion gap, an efficient accumulation and transfer of the lighter fluid fraction (gas) for the upper part of the toroxc3xafdal chamber, or of the heavier fluid fraction (water) for the lower part of the toroxc3xafdal chamber, back towards the toroxc3xafdal chamber can be achieved upon flushing of the extrusion gap by the flushing fluid, such as oil, supplied via the oil duct.
An embodiment of the swivel according to the present invention is characterised in that the length (H) of the third gap section part between the primary seal and the second (horizontal) gap section is between 1 cm and 10 cm.
According to this construction a sufficient volume is present for accumulation and transfer of gas or water being separated from the oil in the seal area. The gas or water will travel up- or downwardly respectively to fill the space above the seal, leaving the seal itself in an oil environment. The lower extrusion gap may be formed as a mirror image of the upper gap, the seal area of the lower gap being immersed in heavy oil which has a greater density than water.
In a further embodiment according to the present invention the lower gap is substantially straight. As the problem of expulsion of oil in the seal area by gas only occurs at the upper gap, the lower gap can be constructed in a relatively simple manner using only axial, or piston-type seals, i.e. seals having their flexible legs parallel to the cylindrical peripheral services of the inner and outer annular walls. For safety reasons a secondary seal may be used behind the primary seal.